Early work in diffraction theory was centered around the study of the interference pattern created by one or more slits in an opaque mask, dependent upon the wavelength of the light, the width of the slits, the separation between the slits and the number of slits. Given the relatively simple geometry involved in these simple patterns, a precise understanding of the patterns involved was readily achievable.
For example, it can be demonstrated both experimentally and mathematically that varying the separation between slits while maintaining the slit width constant will maintain the scale of the overall diffraction pattern envelope while varying the scale of the interference pattern.
However, far more interesting than this is the result of maintaining both slit width and separation between slits constant while increasing the number of slits. Indeed, the discovery of the collective effects of a great number of slits forms the basis for substantially all commercially important diffractive instrumentation. In particular, it can be demonstrated that as the number of slits is increased from two, there occurs a dramatic narrowing of the interference maxima. In addition, as the principal maxima increase in sharpness, additional secondary maxima appear between the principal maxima, with their number increasing with the number of slits. With as few as twenty slits relatively well defined principal and secondary maxima are present.
A principal disadvantage of diffraction gratings comprised of a plurality of slits is the fact that a large percentage of the light which falls on the grating is absorbed by the grating and thus not available for analysis. A partial solution to this problem is the engraving of a plurality of grooves in a reflective substrate in order to form a reflective diffraction grating.
However, the relative intensities of light in the different orders in a grooved grating do not necessarily follow the simple geometric relationships which govern a grating formed by a plurality of slits and whose overall dimensions are much larger than the wavelength. Rather, instead of a relatively simple geometric problem, one must apply electro-magnetic field theory to the particular groove profile in order to calculate the diffraction pattern produced. Nevertheless, regardless of the groove profile, if the distance between the grooves is known, the position of the spectral lines will be the same as in a multiple slit grating. Indeed, in the early days of grating manufacture, the profile of the grating was considered to be largely uncontrollable.
Until World War II, gratings were ruled on so-called "speculum" metal which is a relatively hard alloy of copper and tin. During that period, the manufacture of practical diffraction gratings remained grounded in the mathematical theory developed in the early 1800's and the fabrication of practical devices was substantially limited to refining the work of Professor Rowland who was first able to rule a quality grating after developing a constant screw in the late 1800's.
A major change occurred in the nature of diffraction gratings when the ruling of gratings on aluminum commenced. In particular, it was found that by proper shaping and orientation of a diamond cutting tool it was possible to vary the shape of the profiles in such a way that they would produce a so-called "blaze" of light at any desired angle. Such blazed diffraction gratings form an important class of instruments. Over the years, they have been developed on the basis of field theory and/or iterative fabrication and experimental evaluation. Attention has thus largely been drawn away from transmission gratings and the concentration has been on improving reflection gratings due to the lack of efficiency of transmission gratings.